Introduction

The weight of a newborn mammal, which in normal circumstances is characteristic of the species under consideration, is not simply a reflection of gestation length and/or nutrition but principally is determined genetically through growth rate. Analysis of fetal weight and conception age suggests that mammals can be segregated into three or four groups each having a different growth rate. Irrespective of whether the species displays fast, intermediate or slow growth the overall rate through late embryonic and fetal development is quite smooth, with no noticeable periods of fast or slow growth. There is a gradual decline in the rate from fast in early development to a slower rate as birth approaches (Snow, 1986 for review). Significant departures from the predicted growth curve, either above or below the norm, are regarded as pathological and have a correlation with abnormalities of various sorts (Neligan et al., 1976; Spiers, 1982; Gould, 1986; Jones, Peters & Bagnall, 1986).

What then is known of the controls over embryonic growth?

Maternal influences vs embryonic genotype

It is clear that maternal size is to some extent reflected in fetal size, small mothers tending to have small babies and vice versa. There is also a maternal effect associated with parity, and in polytocous animals an effect of litter size. Part of these phenomena will be a function of the genotype of the fetus and part the physiology of the mother – fetal crowding or undernutrition obviously serving to restrict fetal growth, whether the major controlling factors reside in fetal genotype or in maternal physiology.